Effect of sheep grazing on a leafy spurge-infested Idaho fescue community

  • Published source details Olson B.E. & Wallander R.T. (1998) Effect of sheep grazing on a leafy spurge-infested Idaho fescue community. Journal of Range Management, 51, 247-252.


Leafy spurge Euphorbia esula (native to Europe) and accidentally introduced to USA in the early 1800s, is an aggressive invasive of native upland bluebunch wheatgrass Pseudoroegneria spicata and Idaho fescue Festuca idahoensis dominated grasslands in Northern Rocky Mountain Province (northern Idaho, western Montana, eastern Washington) northwest USA . This study investigated the effect of three consecutive summers of experimental domestic sheep Ovis aries grazing on a leafy spurge infested Idaho fescue community.

In 1992, 1993 and 1994, sheep (Targhee ewes) were rotationally summer-grazed through paddocks in small pastures. Leafy spurge stem and Idaho fescue and bluebunch wheatgrass densities, and frequencies of other species were determined in sample quadrats prior to commencement of grazing in 1992, 1993 and 1994, and in 1995 (9 months after grazing ended). Plant heights were measured each year.
The number of viable leafy spurge seeds in the seedbank was estimated (via soil samples) in 1992 and 1995.

Leafy spurge seedling densities were low in grazed and ungrazed areas in 1992 and 1994, but lower in grazed compared to ungrazed areas in 1993 and 1995. Grazing did not increase or decrease mature leafy spurge density over the four study years (1992-1995). However, the number of viable spurge seeds in the seedbank was lower in 1995 than in 1992, this reduction being greater in grazed than ungrazed areas.
Grazing increased Idaho fescue density whilst bluebunch wheatgrass density declined. Kentucky bluegrass Poa pratensis, Sandberg bluegrass Poa sandbergii, annual bromes Bromus spp. and sedge Carex spp. frequencies increased in grazed areas. There was little effect on cool season native grasses (whose main growth periods are in spring and autumn), possibly because the site was grazed primarily in midsummer when they are dormant.
As thee consecutive years of grazing reduced numbers of viable leafy spurge seed, the authors suggest that longer-term sheep grazing may help in spurge control. It is emphasised that grazed sites should be monitored to ensure that grazing is not directly detrimental to native species, and that other undesirable species do not increase at the expense of the native grassland community.
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